When picking a hash function you also need to be worried about "hash distribution."

If you open up a doctor's office in South Boston, you might have an issue ordering charts by last name only. Your M cabinet will be overflowing.¹

For that scenario, using the first letter of the last name is a non-ideal hash function because when so many people have last names starting with the same letter, they will not be evenly distributed amongst the buckets.

Making a hash function with a good distribution is hard. Objects.hash will do a decent job of it and thats why we use it.